The design and fabrication of electronics frequently involve bonding electronic components to circuit boards using such materials as metal solders and epoxy-based adhesives. Metal solders comprise metal alloys, such as lead-tin alloys, that may be melted at a low, near-eutectic point temperature and applied to intended adherends to achieve adhesion therebetween upon solidification by cooling. Metal solders are limited in application to joining metals together and are therefore ineffective in joining other broadly-employed materials such as plastics. Moreover, the use of lead in metal solders is undesirable, since lead is toxic and is highly regulated in use. Other disadvantages to metal solders include their use of fluxes to promote melt and flow, which must then be removed with solvents such as containing environmentally-toxic chlorofluorocarbons (CFCs), and the tendency of solder-plated traces and pads to oxidize over time, leading to potential losses in solderability.
In comparison to metal solders, epoxy-based adhesives demonstrate the ability to strongly adhere to a variety of materials, including metal, glass, plastic, wood, and fiber, and consequently are often used to bond dissimilar materials. Further, epoxy compounds are known to exhibit excellent resistance to attack by many corrosive chemicals. In contrast to metal solders, epoxy-based adhesives contain no toxic metals, require no fluxes which would require removal, and may be used on non-solder-plated traces and pads including passivated or noble metals.
Presently, electrically-conductive epoxy-based adhesives are available in two forms: two-component systems and one-component systems. Two-component epoxy-based adhesives are readily curable at room temperature but are inconvenient to use and store. The components of two-component systems must be accurately measured and properly mixed immediately prior to use. Thus, the various components to be mixed must be separately stored until use, and production workers are charged with the added responsibility of preparing epoxy-based adhesives having uniform properties. It follows that two-component epoxy-based adhesive systems are not favored.
One-component epoxy-based adhesives are available for industrial application in various forms, such as rigid epoxy adhesives, frozen pre-mix flexible epoxy adhesives, and room-temperature stable flexible epoxy adhesives. While such adhesive compositions are conveniently stored as a single component, they require curing at elevated temperatures. Rigid epoxy adhesives include such compounds as bisphenol-A epoxy resins and novolac resins. These rigid epoxy adhesives exhibit strong adhesion for many materials and may be conveniently stored at room temperature. However, rigid epoxy-based adhesives form brittle bonds that are often insufficiently pliant for bonding dissimilar materials or for withstanding mechanical shock or vibration. For example, a brittle bond between dissimilar materials with different thermal expansion rates may be unable to withstand the stresses caused by the thermal mismatch, so that both the bond and its adherends may be susceptible to failure. Bonds formed with rigid adhesives may not be sufficiently drop-resistant to withstand even relatively minor mechanical shocks, such as deriving from a drop or impact.
While flexible epoxy adhesives are available, namely as frozen pre-mix epoxy adhesives and room-temperature stable epoxy adhesives, they tend to be weak and friable. Therefore, none of these adhesives has sufficiently high elongation and tensile characteristics such that their resultant bonds withstand even relatively minor stresses when the bond surface area is small. An example of an application employing small bond areas is the interface between the component lead of fine-pitch surface mount components and the circuit trace or pad. Thus, while flexible epoxy adhesives are commercially available, their bonds remain unacceptably susceptible to failure from minor stresses, such as mechanical shock deriving from a drop or impact.
With regard to frozen pre-mix adhesives, a description of one such adhesive is found in U.S. Pat. No. 4,866,108, assigned to the same assignee as the present application, which discloses and claims a flexible epoxy adhesive comprising a mixture of a fatty acid modified epoxy resin and an oxypropylene polyether epoxy resin in the ratio of about 1:3 to 1:1. The adhesive also includes a stoichiometric amount of a polyamine curing agent, 1 to 20 total liquid weight percent of a plasticizer, 50 to 80 weight percent of aluminum oxide, and 1 to 5 weight percent of microfine silicon dioxide particles. Another frozen flexible epoxy adhesive is disclosed in U.S. Pat. No. 5,367,006, entitled "Superior Thermal Transfer Adhesive" and assigned to the same assignee as the present application, and is directed to an adhesive comprising an aliphatic epoxy resin, an aliphatic polyamine curative, a thixotropic agent, and a filler. While these frozen, pre-mix epoxy adhesives exhibit low glass transition temperatures within the range of about -10.degree. C. to -60.degree. C. and good adhesion, they are not electrically conductive nor do they possess the drop-resistance necessary to withstand relatively minor impacts when bond areas are small. Moreover, the adhesives of '108 contain diglycidyl ester, which is not hydrolytically stable, such that those adhesives are unsuitable for high-reliability terrestrial applications, and the adhesives of '006 contain cardonal-based resin which have unacceptably high levels of hydrolyzable chloride.
A room-temperature stable flexible epoxy adhesive that is electrically conductive is disclosed and claimed in U.S. Pat. No. 5,575,956 entitled "Room Temperature Stable, One Component, Electrically-Conductive, Flexible Epoxy Adhesives", which is assigned to the same assignee as the present application. The adhesive formulations of that application comprise an electrically-conductive filler comprising a metal and a polymer mixture comprising (i) at least one polyepoxide resin having a hardness not exceeding a durometer Shore D reading of 45 when cured with a stoichiometric amount of diethylene triamine, and (ii) a substantially stoichiometric amount of at least one latent epoxy curing agent. While this adhesive formulation exhibits low glass transition temperatures on the order of -50.degree. C. and good adhesion, it also does not possess the drop-resistance necessary to withstand relatively minor impacts when bond areas are small.
An encapsulant composition employing epoxy resins is disclosed and claimed in U.S. Pat. No. 5,457,165, entitled "Encapsulant of Amine-Cured Epoxy Resin Blends" and assigned to the same assignee as the present application. The encapsulant comprises (a) a first liquid epoxy resin selected from the group consisting of (i) the diglycidyl ether of polyoxypropylene glycol, and (ii) the diglycidyl ester of linoleic dimer acid; (b) a second liquid epoxy resin comprising the diglycidyl ether of 1,4-butanediol present in the amount ranging between 12 to 55 parts by weight of the composition; and (c) a stoichiometric amount of an epoxy resin curing agent selected from the group consisting of a flexibilized polyamine and a flexibilized polyamide. The cured encapsulant product offers thermal and hydrolytic stability and is insulative and adhesive. However, this encapsulant composition is not electrically conductive and thus cannot serve in applications requiring this characteristic.
In sum, while various adhesives are commercially-available that offer desirable qualities such as flexibility, high elongation and tensile characteristics, strong adhesion, and electrical conductivity, none of the commercially-available adhesives offer drop-resistance. Drop-resistance is important because such commonly-handled (and therefore, commonly-dropped) devices such as telephones, calculators, and laptop computers contain components that are surface-mounted onto wiring boards with narrow bond thicknesses that may be too weak and/or too rigid to withstand drops and minor impacts.
Thus, a need remains for a flexible electrically-conductive epoxy-based adhesive exhibiting drop-resistance while retaining high elongation and tensile characteristics, resiliency, excellent adhesion, and ease of reworkability.